1. Field of the Present Invention
This invention relates generally to a lock-up type automatic transmission system provided with a hydraulic fluid cooling device.
2. Description of the Related Art
It is a recent trend in the automobile industry to adopt lock-up type automatic transmission systems for improving the transmission efficiency and achieving an enhanced fuel economy. The lock-up type automatic transmission system typically includes a torque converter which is interposed in a power transmission path, a lock-up clutch defining an apply chamber and a release chamber in the torque converter, and a lock-up control device for achieving a lock-up state of the torque converter in which input and output elements of the converter are directly connected with each other. The lock-up state is established when a vehicle is driven in a lock-up region in which the torque multiplying function and the torque fluctuation absorbing function of the torque converter are unnecessary.
When performing a lock-up control operation on the torque converter of such a type, similarly as in the case of an automatic transmission described in "Service Manual For Nissan RE4R01A-Type Full-Range Electronically Controlled Automatic Transmission" issued by Nissan Motor Co., Ltd., the assignee of the entire interest and title relative to the present invention, it is first judged which of a lock-up region and a converter region the state of a driven vehicle belongs to. Subsequently, if belonging to the lock-up region, the torque converter is put into a lock-up state in which the input and output elements are connected to each other. Conversely, in case of the converter state, the torque converter is put into a converter state in which the direct connection is not established between the input and output elements.
As described in the abovementioned service manual, in a conventional apparatuses for performing the lock-up, it is the general practice to establish the lock-up state of the torque converter by means of a lock-up control valve, through which a torque converter operating pressure is supplied to the apply chamber and the release chamber is brought into communication with a drain circuit.
Besides, an automatic transmission is provided with an oil cooler for cooling the hydraulic fluid thereof. A conventional hydraulic fluid cooling device of an automatic transmission system, including an oil cooler as a primary component, is disclosed in the abovementioned service manual, and has an arrangement as described below.
FIG. 1 illustrates a conventional hydraulic fluid cooling device provided in an automatic transmission system. Reference numeral 1 denotes a torque converter; 2 a lock-up control valve; and 3 an oil cooler. The torque converter 1 has an apply chamber 1.sub.A and a release chamber 1.sub.R which are partitioned by a lock-up clutch (not shown). When a torque converter operating pressure P.sub.T is supplied to the apply chamber 1.sub.A under the control of the lock-up control valve 2 responsive to a pressure signal sent from a lock-up solenoid 4, the torque converter 1 is put into a lock-up state by the engagement of a clutch. Conversely, when the torque converter operating pressure P.sub.T is supplied to the release chamber 1.sub.R, the torque converter is put into a converter state by releasing the lock-up clutch. Further, a hydraulic fluid cooling device is formed by connecting the oil cooler 3 through an orifice 5 with a circuit for supplying the torque converter operating pressure to the apply chamber 1.sub.A.
It has been confirmed, however, that in the case of the conventional hydraulic fluid cooling device for a lock-up type automatic transmission system having the abovementioned arrangement, the following problems may arise from the fact that the hydraulic fluid of an amount corresponding to the cross-sectional area of the orifice 5 is moved to the oil cooler 3 during the torque converter 1 is in a lock-up state.
Namely, for improving the fuel economy by the locking-up of the torque converter, it is necessary to enlarge the lock-up region up to low load on low speed driving of a vehicle to the extent possible during operation thereof. Meanwhile, when the vehicle is running at a low speed, the engine speed becomes low. Moreover, the torque converter uses discharged from an engine-driven oil pump, as the hydraulic oil for various operations including the lock-up control operation. Thus, in case of such a low speed of the vehicle, reduction in amount of the oil discharged from the oil pump makes it difficult to accomplish the locking-up of the torque converter. Therefore, when initially setting the lock-up speed of the vehicle, it has been a common practice to set the lock-up speed to the very limit speed at which the locking-up of the torque converter can be accomplished.
Nevertheless, if the hydraulic fluid of the amount corresponding to the area of the orifice 5 is always supplied to the oil cooler 3 during the locking-up of the torque converter 1, as in the case of the conventional hydraulic fluid cooling device, the hydraulic fluid may be insufficiently supplied to the apply chamber 1.sub.A. This may result in occurrence of a situation where the torque converter cannot be completely locked-up at the initially set lock-up speed of the vehicle. Consequently, this may raise a problem that the lock-up speed of the vehicle cannot be sufficiently reduced.